1. Field of the Invention
This invention is related to circuit interrupters and more particularly to current limiting circuit interrupters wherein a movable contact support arm and a stationary contact support arm are disposed within a slot formed in a magnetic contact arm driving device.
2. Description of the Prior Art
It is common in the prior art to provide for current limiting during circuit fault conditions. A common method of providing current limiting is through the use of current limiting fuses in combination with a standard stored energy type circuit interrupter as disclosed in U.S. Pat. No. 3,077,525 to Dyer. U.S. Pat. No. 3,815,059 to L. A. Spoelman discloses a circuit interrupter in series with an electromechanical current limiting device which utilizes a force generated by the overload current to drive the movable contact arm open. In U.S. Pat. No. 3,815,059 the current limiting interrupter was provided with a movable contact arm which was pivoted around one end and which moves into the slot of a magnetic drive device during circuit interruption.
When dealing with current limiting circuit interrupters the term I.sup.2 t is often utilized to describe the effectiveness of a particular device. The meaning of I.sup.2 t can best be understood by considering an electrical source feeding a simple resistive load through the circuit interrupter. The total energy seen by the load is given by: current (I) flowing through the load times the voltage drop (IR) across the load times the time (t) current flows. This can be expressed mathematically as energy dissipated equals RI.sup.2 t. During normal operation, this energy RI.sup.2 t is easily dissipated by the system. When a fault current occurs, however, current (I) can become very large in value. For example, with present day power supplies possible fault current up to 100,000 amps on a 100 amp breaker can occur. During fault conditions the value of the load, R, will remain approximately the same but the total energy input into the system will be very large (R [fault current].sup.2 t). In a practical system, if the load is to be protected, the fault current should be limited to as small a value as possible. A reduction in the time of fault current flow, t, will give a further reduction in the energy input into the load. This means that in any current limiting device the total I.sup.2 t i.e., (the integrated fault current).sup.2 .times. (time it flows) is the important parameter to measure. The smaller this I.sup.2 t value the better the performance of the current limiting device.
In the circuit interrupter art, it has long been recognized that it is of a distinct advantage to provide fast interruption of an established arc. It is well known by those skilled in the art that it is desirable to affect a rapid extinction of the arc as quick as possible, inasmuch as the fault current flowing through connected electrical equipment will damage the equipment unless the fault current is limited. Due to heating, voltage surges, and the like it is desirable to affect extinction of the fault current arc as quick as possible.
It is also desirable to simplify the circuit interrupter structure to bring about cost reductions. Since circuit interrupter manufacturing is normally conducted on a high volume basis, cost and simplicity of the device is an important advantage. Likewise, the ability to rapidly reset and reuse the current limiting device is desirable, since resettable circuit interrupters have many well-recognized advantages over fused devices.